from PyQt5.QtWidgets import QPushButton, QWidget, QApplication, QHBoxLayout
from PyQt5.QtCore import QObject


class Button(QPushButton):

    def __init__(self, callback):
        super(Button, self).__init__()

        self.setText("Hi")

        #1
        self.clicked.connect(lambda: callback())

        #2
        self.callback = callback
        self.clicked.connect(self.callback)


class Example(QObject):
    def __init__(self):
        super(Example, self).__init__()

        self.btn = Button(self.callback)

    def callback(self):
        print("callback")


class Widget(QWidget):
    def __init__(self):
        super(Widget, self).__init__()

        ex_ = Example()

        h_box = QHBoxLayout()

        h_box.addWidget(ex_.btn)

        self.setLayout(h_box)

        self.show()


if __name__ == "__main__":
    import sys


    app = QApplication(sys.argv)
    wd = Widget()
    sys.exit(app.exec_())